Seed Specifics

By Peter Loewer | December 31, 2001

Fortunately, a Ph.D. in botany or genetics isn't necessary to grow plants
successfully from seed. But this chapter on seed basics will help you better
understand what seeds are, where they come from, their role in plant
reproduction, and why they are so critical to the health and survival of plant
species. It will also help you appreciate—the next time you head for the
pumpkin patch, perennial r, fields, or woods—the wonders of
these typically tiny parcels of life, which contain all the fundamental parts
of a mature plant—leaves, stem, and root.

A complete, or perfect, flower contains both a female part—the pistil,
usually composed of a stigma, style and ovary—and male parts—stamens,
usually composed of a filament and an anther. An incomplete flower is either
male or female.

And Then There Were Seeds

Plants didn't always produce seeds. Millions of years ago, when the world
was mostly water, swamp, or just plain wet, terrestrial spore-producing plants
like ferns and mosses were supreme because they used water to facilitate
reproduction. Once the continents began to drift apart and land rose, the
earth's climate began to fluctuate. The seasons were born. Seed-bearing plants
were one adaptation to these drier conditions. Scientists divide these higher
plants into two groups: the gymnosperms (gymno, meaning naked, and
sperm, meaning seed), such as pines, firs, and cycads; and the
angiosperms (angio, meaning contained in a vessel), or flowering plants.
Seed-bearing plants, which are of most interest to the majority of gardeners,
represent one of the most important steps in the evolution of the plant
kingdom.

The life cycle of a gymnosperm is clearly represented by the white pine,
Pinus strobus. Every spring, pollen-bearing male cones appear at the
ends of the tree's lower branches, clustered just below the new crop of
needles. When mature, the cones release clouds of pollen, which are carried by
the air to the female cones growing at the trees' tops. After being fertilized
by the pollen, the individual eggs mature into embryos. That process takes a
minimum of 13 months, not including an additional year or so for the seeds to
fully develop. Finally, mature and winged, the seeds gently glide to the
ground, where they will lie through winter, waiting for spring's warmth to
prompt them to germinate.

Fabulous Flowers

About 75 percent of all the seed-bearing plants on earth today are not
gymnosperms, but rather angiosperms. All angiosperms produce flowers, which
botanists define as shoots, modified for reproduction." Although gardeners
value flowers for their shapes, colors, and fragrance, nature designed them
solely as a means of reproduction. As a result, most flowers have similar
elements.

First, the typical flower has a receptacle, a structure that holds
the rest of the floral parts together. Before opening, the flower petals are
protected by a calyx, an outermost whorl, or ring, made up of modified
leaves called sepals. A second inner whorl, called the corolla,
is made up of petals. Collectively, the calyx and corolla are known as the
perianth of a flower.

Petals are often brightly colored in order to attract insects and birds (a
few flowers, such as the poinsettia, appear to have brightly colored petals
that actually are modified leaves called bracts; in a handful of
flowers, such as tulips, these colorful parts are called tepals, because
it's difficult to determine if they are sepals or petals). Petals can serve as
both beacons and landing fields for insects, and many petals even have lines on
the surface that act as arrows, leading pollinators to the flower's center.

Inside the whorl of petals are the stamens, the male parts of the
flower (see illustration, above). The typical stamen
consists of a long stalk, or filament, with a swollen tip called the
anther, which is the structure that produces grains of pollen. The
female part of the flower, known as the pistil, usually includes the
stigma, the sticky portion that traps pollen; the style, the
stemlike portion that holds the stigma where it can best catch pollen; and the
ovary, the swollen base of the pistil. Inside the ovary are immature
seeds, or ovules, waiting to be fertilized.

When flowers have some form of all these parts, they're known as complete.
Complete flowers are also known as perfect flowers, because they contain both
male and female parts; imperfect flowers have either male or female parts,
either stamens or pistil, but not both. Plants, such as sweet corn, squash, and
cucumbers, that contain both male and female flowers are called monoecious.
When the male and female flowers occur on different plants, as in hollies,
asparagus, and persimmons, the plants are called dioecious; to produce
flowers—and seeds—you need to have both a male and a female plant in
your garden, or at least in the immediate vicinity.

Green Genes

Flowers usually must be pollinated for seeds to form. There are two basic
kinds of pollination: self-pollination and cross-pollination. Self-pollination
occurs when the pollen of a flower fertilizes that same flower or another
flower on the same plant. The typical self-pollinating species has perfect
flowers, the ones with both male and female parts. Peas, lettuce, tomatoes,
snap beans, and snapdragons are examples of self-pollinating plants; they can
be grown from seed without fear of crossings that may result in plants with
unwanted variations from the parents.

Cross-pollination results when pollen from one flower fertilizes a flower on
another plant. The flowers of cross-pollinating plants can be either perfect or
imperfect. The constant mixing of genes that occurs with cross-pollination is
crucial in helping species remain healthy and vigorous.

After centuries of observation, we know that cross-pollination is carried
out naturally by several types of pollinators. The most important pollinators
are the wind, insects, birds, bats, and, finally, water. Wind-pollinated plants
normally have no nectar, no fragrance, and no brilliant colors to attract
wildlife. Instead, their floral structure is suited to sending and receiving
pollen on the breeze. Grasses are a good example; although their flowers are
visible, they are tiny.

Plants that are pollinated by insects, birds, bats, and other wildlife tend
to have bright, attention-grabbing flowers—real advertisements for
themselves. The payoff for the pollinating animals is food: either pollen,
which is eaten by some insects, or nectar. Many flowers are specifically
designed to accommodate the animals that will pollinate them. The foot-long
spurs of the Christmas star orchid depend on a specific moth with a foot-long
tongue; the stink of carrion flowers attracts the tiny flies that transport its
pollen; flowers pollinated by bats tend to open at dusk, the same time that
bats become active.

We humans are also adept at a specific type of cross-pollination, known as
hand-pollination. The objective of hand-pollination is to avoid random
cross-breeding—and thus to guarantee that the seeds you save will produce
plants like their parents. To insure this, you must prevent insects from
visiting the flowers of plants you've selected for seed saving. Then you must
perform the insects' job yourself.

Sex To Seeds

Imagine it's a bright sunny day, somewhere in the temperate zone. The sun is
high in a blue sky and a breeze moves across a field of waving grasses and
brightly colored wildflowers. You stop at a clump of sundrops, a day-flowering
species in the evening primrose family. On top of a two-foot stem, bright
yellow, four-petaled flowers are in full bloom. From stage right, enter a
bumble bee, his hind legs bright yellow from pollen. The bee buzzes in flight,
then spots the flowers. It swoops down and sticks its tongue into the
four-pronged stigma at the flower's center. As the bee moves about in a
somewhat clumsy way, pollen grains attach themselves to the stigma.

What happens next takes place on the microscopic level. When a grain of
pollen becomes attached to the stigma, one of its two cells stimulates the
creation of a pollen tube that grows inside the style, creating a path between
the stigma and the flower's ovary. Once the tube is complete, the second cell
divides into two sperm, which use the pollen tube to reach the ovule. One sperm
joins with an egg to create a zygote, a fertilized egg; the second sperm fuses
with other nuclei to form the endosperm, a food supply contained in the
seed.

As time passes, the zygote becomes the embryo of the new plant. The seeds of
flowering plants come in a great variety of sizes, shapes, and textures, but
the embryonic plants contained within have the same basic design, with a
rudimentary leaf or leaves, called cotyledons, as well as a root tip and a
stem. (Cotyledons are the first leaves to appear after a seed sprouts; they are
followed by the plant's true leaves.) Most flowering plants have two
cotyledons, and thus are called dicotyledons, or dicots. The monocotyledons, or
monocots, a smaller group, produce only one seed leaf when they sprout. Plants
in the grass family, including corn and grains, are monocots.

In flowering plants, the seed is encased in the ovary, which enlarges into a
fruit. The fruit may be red like an apple or a rose hip, green like a cucumber,
purple like an eggplant, or brown like an acorn. In plants like tomatoes, the
fruits become increasingly soft and fleshy as their seeds mature; in other
plants, such as garden peas or winter aconite, the fruit wall becomes
increasingly dry and eventually splits. Oriental poppies form hard, dry fruits
that disperse seeds through small openings like a salt shaker's; other species,
such as cherries, have a soft fruit that surrounds a hard pit, or stone, which
contains a seed.

No matter what size or shape fruit it is encased in, once the seed, with its
embryonic plant, has formed completely, growth stops and the seed typically
enters a period of dormancy. The tiny plant consumes minute amounts of energy
from food stored inside the seed—just enough to keep it alive, until it
germinates and grows.

Spreading The Treasure

After the development of the seeds and fruit, the next order of business for
any flowering plant is dispersal—getting the next generation out into the
world where it can find suitable habitat to germinate and grow. Anyone who has
ever tramped through the woods with a longhaired dog or wearing wool pants
knows how some seeds travel: They become affixed to clothing and fur by using
hooks, barbs, and even Velcro-like natural structures to guarantee their
dispersal around the country, if not the world. The seeds of mistletoe are
covered with a sticky substance and become attached to birds and other animals.
We humans unintentionally spread seeds in other ways: as discarded foodstuff or
as unintentional traveling companions, hidden in pants cuffs, or stuck to the
mud on dirty boots and shoes—and we spread them intentionally by buying or
collecting seeds and planting them in our gardens.

Plants don't require humans to spread their seeds, however. Small seeds
often use the wind to blow them from place to place. Some seeds, like those of
orchids, are dustlike; others have plumes, like dandelion or milkweed; or, as
do the seeds of maple, they have wings that enable them to glide through the
air.

Whether by rain, streams, or ocean currents, many seeds are distributed by
water. Very small seeds, especially if they are light in proportion to their
size, float. Corky seeds, like those of the carrot family, can stay afloat for
weeks.

Seeds are also spread by animals that eat fleshy fruits. The seeds in these
fruits often pass through animals' alimentary tracts unharmed; in some cases,
the journey through the digestive system even speeds up germination. Beetles,
ants, and scores of other insects carry seeds from one location to another.
Finally, plants themselves are responsible for spreading seeds: A number of
species, such as the yellow-flowered creeping oxalis, shoot their seeds into
the air. The record is probably held by the West Indian swordbean, whose large
pods snap open with a crack and propel their seeds at least 20 feet. Less
impressive is the squirting cucumber, an annual vine whose fruits push their
seeds out in a lump of semi-liquid mucilage. Not neat but effective.

Seed Steadfastness

Wherever they land, seeds can remain in suspended animation until conditions
are right to spur germination. While some seeds, given water, will germinate
immediately, many others follow an internal clock which insures, as much as
possible, that when the seed does germinate, conditions will be conducive to
its growth. For example, some seeds require a period of exposure to cold
temperatures to break dormancy -- nature's way of making sure that seeds of
plants in the temperate zone don't germinate until the killing cold of winter
has passed. Gardeners need to simulate these natural conditions in order to get
such seeds to grow. (See "Special Handling")

The seeds of some plants can remain dormant for months or even years. While
the seeds of most species remain viable for only a few years (some for only a
few days), others have remarkable longevity. Although many of the tales of
sprouting seeds that were found in centuries-old tombs are apocryphal, there is
good evidence that some seeds have astonishing durability. The record for
longevity probably goes to an Arctic lupine from the Canadian Yukon—seeds
germinated after they had been frozen and buried in an ancient rodent burrow
for 10,000 years.

Scientists believe that the seed coat is the mechanism that allows a seed to
be viable for so long. Typically, this structure consists of an outer and inner
cuticle, often impregnated with waxes or fats, which are surrounded by one or
more layers of thick-walled, protective cells that are so hard that it is
difficult for water to seep into the interior and trigger germination.
Sometimes this makes the gardener's job more complicated, requiring special
measures to prompt a seed to germinate (some seeds won't germinate until the
protective seed coat has been roughed up a bit, as it is when seeds pass
through an animal's gut). But mostly, it's a boon, because seeds often must be
stored for weeks or months, and in some cases years, before they can be
planted.

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